This concerns a method and apparatus for using an adaptive equalizer and, in particular, the digital filter section of the equalizer for synchronizing a data communication receiver with a received signal.
Equalizers are commonly used in modem receivers to compensate for amplitude and phase distortions in communication lines. Equalizers may be fixed or adjustable and the adjustable equalizers may be manual or automatic. Automatic equalizers are often referred to as automatic regulators. Adjustable equalizers or regulators may be open or closed loop devices, the closed loop or feedback regulator being designed to respond to a signal on the communication line. See, for example, J. G. Proakis, "Adaptive Digital Filters for Equalization of Telephone Channels", IEEE Transactions on Audio and Electroacoustics, Vol. AU-18, No. 2, p. 195 (June 1970); Bell Telephone Laboratories, Transmission Systems for Communications, pp. 373-395, 722-723 (Rev. 4th Ed. 1971). An adaptive equalizer typically comprises a tapped delay line through which the received signal is shifted, a plurality of taps in the delay line, means for modifying the amplitude and/or phase of the signal at each tap, and means for summing the modified signal produced by each modifying means. Typically, the modifying means are multipliers which multiply the signals at the taps with coefficients selected to correct for amplitude and phase distortions in the received signal that is shifted through the tapped delay line.
Some type of synchronization between the receiver and the signal it receives is absolutely necessary for the receiver to properly interpret the received signal. At a minimum, the receiver modem must establish baud synchronization which requires the baud rate at the receiver to be exactly the same as that in the data being received and properly phased with such data. In addition, some receiver implementations, e.g., coherent detectors, require carrier synchronization and preamble synchronization. Ideally, for carrier synchronization the local carrier signal is at the same frequency and properly phased to the transmitter carrier signal that modulates the signal being received although it is recognized that the effect of carrier synchronization can be achieved without actually synchronizing signals.
For preamble synchronization a locally generated training sequence must be in phase with a preamble training sequence which is located at the beginning of the received data signal. Preamble synchronization is either exact, i.e., established within one baud time or approximate, i.e., within an acceptable tolerance for the receiver to properly synchronize. Exact synchronization permits superior training capability. To achieve exact preamble synchronization in the prior art, it is customary to insert a leading edge timing signal at the beginning of the transmitted signal and to employ suitable apparatus in the receiver to detect such a timing signal. As will be apparent, the production and detection of such a signal requires special apparatus and the added costs and complexity inherent therein. As a result, not all receiver modems are designed to achieve exact carrier synchronization or preamble synchronization. For example, the Bell System 208 modem does not provide a timing event but relies on the receiver to correctly detect a four-phase sequence in the training sequence in order to establish equalizer training. Such approximate permeable synchronization, however, depends on the ability of the communication channel to transmit the four-phase training sequence error free. Beyond a certain degree of channel amplitude and phase distortion, errors are introduced and adaptive equalization can not be established.
I have found that portions of an adaptive equalizer can be used for exact preamble synchronization by modifying the digital filter section to recognize a pre-determined signal in the received signal. This may be accomplished by applying to the modifying means of the digital filter input signals which are proportional to the complex conjugate of signals in the received signal. When the signals in the tapped delay line are aligned with this complex conjugate, the output of the summing means will be discernibly greater than otherwise. As a result, a threshold device can be used to recognize the exact baud time the preselected signals in the received signal are contained in the tapped delay line. The output of such threshold means can then be used in conventional fashion to establish exact preamble synchronization between the receiver and the received signal.